Power converter package and thermal management

ABSTRACT

Power conversion apparatus can include a circuit board with power conversion circuitry and a package. The package may be formed by encapsulating areas of the circuit board assembly either before or after the interface contacts are attached to the circuit board. A method for encapsulating two sides of a substrate can include providing a mold that fills a larger first cavity to create a sealing force on a smaller second cavity. The encapsulant flows through the first cavity into the second cavity. A thermal extender can include a surface for mounting a heat dissipating power converter and a surface for mating with an external circuit board. Interface conductors may mate with contacts on the heat dissipating power converter and with conductive regions on the external circuit board. A heat sink may be thermally coupled to remove heat generated by the power converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. application Ser. No. 10/303,613, filed on Nov. 25, 2002, which isherein incorporated be reference in its entirety.

TECHNICAL FIELD

This invention relates to power converters, and more particularly topackaging of and thermal management in power converters.

BACKGROUND

Electronic power converters accept electric power from an input sourceand convert it into a form suitable for use by a load. As definedherein, power converters are devices that convert electric power from anAC source or a DC source to deliver it to a load at an AC voltage or aDC voltage while providing some of the following functions: voltagestep-up, voltage step-down voltage regulation, with or without galvanicisolation. Examples of power converters include DC-DC converters,switching regulators and active filters.

The power density of a power converter as defined herein is the fullrated output power of the power converter divided by the volume occupiedby the converter. Trends in contemporary power conversion have resultedin dramatic increases in power density of marketable power converters.Prior to 1984, power densities were typically below 10Watts-per-cubic-inch. In contrast, power densities greater than 500Watts-per-cubic-inch have become possible today. A very high density,galvanically isolated, point of load DC-to-DC transformer, called a“VTM” is described by Vinciarelli in U.S. patent application Ser. No.10/066,418, filed on Jan. 31, 2002, entitled “Factorized PowerArchitecture With Point of Load Sine Amplitude Converters,” and in theCIP application Ser. No. 10/264,327, filed Oct. 1, 2002 (the “FactorizedApplication”).

The current density of a power converter is defined herein as its fullrated output current divided by the board area occupied by theconverter. Escalating current requirements of microprocessors (CPU's),now approaching 100 Amperes, and the need to provide such currentswithin a small footprint in close proximity to the CPU has gone beyondthe capacity of contemporary power supply technology. Commerciallyavailable solutions are characterized by a current density of less than10 A/in2 and are inadequate to support future CPU requirements. SineAmplitude Converters, of the kind described in the FactorizedApplication ibid, are capable of providing the low voltage requirementsof future microprocessors with current densities exceeding 50 A/in2.They utilize a two-sided circuit board assembly including transformercore structures protruding from both sides of the circuit board. Outputcurrents in excess of 50 Amperes need to be carried from the converter'sPC board, at one elevation, to the CPU board, at a different elevation.These interconnections need to be made with low resistance andinductance, consistent with the current slew rate requirements of ahighly dynamic load.

Power converters dissipate heat in operation. Increases in power densitymake thermal management more difficult, particularly where the increasein power density exceeds the corresponding increase in efficiencycausing a net increase in heat density. Thus, advancements in powerconversion technology may often present significant challenges in termsof thermal management technology. These challenges impose constraints onthe packaging architecture used to house the converter and its input andoutput terminals: the package must exhibit low thermal resistancebetween its internal hot spots, particularly its semiconductorjunctions, and external heat sinks. Depending on the specific thermalenvironment surrounding the power converter, it is desirable to removeheat from the converter package through its case and/or terminals. Lowjunction-to-case and junction-to-terminal thermal resistances arerequired to keep internal temperature rises acceptable. And the need fora good thermal interface must not interfere with the need for flexiblemounting of the power converter package, while respecting constraintsassociated with mechanical tolerances of the converter package and ofthe system with which the converter is coupled.

One way to mount a high-density power converter, shown in FIG. 1, isdescribed in Vinciarelli et al, U.S. Pat. No. 5,526,234, “PackagingElectrical Components” (assigned to the same assignee as thisapplication and incorporated by reference). In the Figure, steps on thecase of a power converter 10 allow the upper wall of the converter 12 tolie within a hole 14 in circuit board 16. The effective height of thecombined power converter package and circuit board is reduced because aportion of the height of the package is coextensive with the thicknessof the circuit board 16. Thermal management is enhanced because both theupper and lower surfaces 12, 13 of the power converter are exposed forheat removal (e.g., by use of forced air or by heat sink attachment).The power density of power converter 10, on a stand-alone basis, is thefull rated output power of the converter divided by the total volumeoccupied by the converter. However, the equivalent power density of theconverter, when mounted as shown in FIG. 1, is higher than thestand-alone power density because a portion of the height of the powerconverter package is coextensive with the thickness of the circuit board16 and the incremental volume occupied by the converter above and belowthe circuit board 16 is less than the total volume of the stand-aloneconverter.

SynQor, Inc., Hudson, Mass., USA manufactures DC-DC power converters andDC transformers which comprise components mounted on both sides of aprinted circuit board and magnetic components which pass throughapertures in the printed circuit board and pins for connection toanother circuit board. One such converter, called a “BusQor™ BusConverter,” is described in data sheet “Preliminary Tech Spec, NarrowInput, Isolated DC/DC Bus Converter,” SynQor Document No. 005-2BQ512J,Rev. 7, August 2002.

Vinciarelli et al, U.S. Pat. No. 6,031,726, “Low Profile Mounting ofPower Converters with the Converter Body in an Aperture” (assigned tothe same assignee as this application and incorporated by reference)describes power conversion apparatus in which a power converter 20extends through an aperture 21 in a circuit board 23. One suchembodiment is shown in FIGS. 2A through 2C. In the figures, the powerconverter 20 is mechanically and electrically connected to a terminalboard 22 and power and signal inputs and outputs are routed, viaconductive runs and solder connections, from contact pads 26 on theterminal board to contact pads 24 which extend from the power converterbody. A heat sink 27 surrounds the outside of the power converter to aidin heat removal. The length, L2, of the terminal board 22, is greaterthan the length, L1, of the aperture 21 in the circuit board 23. Thecontact pads 26 are connected by solder (not shown) to runs 25 on thecircuit board 23. Because a portion of the body of the power converter20 is coextensive with the circuit board 23, the equivalent powerdensity of the power converter is greater than the stand-alone powerdensity, as explained above with respect to FIG. 1.

A power conversion apparatus, in which a power converter is mounted inan aperture in a circuit board, and in which a compliant connectionscheme along the sides of the power converter allows for variation ofthe extension of the power converter within the aperture, is describedin Vinciarelli et al, U.S. patent application Ser. No. 09/340,707, filedon Jun. 29, 1999, and entitled “Mounting Electronic Components onCircuit Boards.” A power conversion apparatus, in which a powerconverter is mounted in an aperture in a circuit board, and in which atleast four sides of the power converter, including the two sides whichlie entirely above and below the surfaces of the circuit board, arecovered with heat sinks to aid in the removal of heat from the powerconverter, is described in Vinciarelli et al, U.S. Pat. No. 6,434,005,“Power Converter Packaging” (assigned to the same assignee as thisapplication and incorporated by reference).

Takatani, Japan Patent 2-142173, “Integrated Circuit Part and MountingStructure Thereof” describes an assembly 450, shown in FIG. 18,consisting of a pair of over molded integrated passive networks 452, 453connected by leads 454 to circuit etches (not shown) on both sides of asubstrate 456. As shown in the Figure, the assembly may be mounted overan aperture 458 in a printed circuit board 460 so that the over moldedintegrated passive network 453 on one side of the substrate pass intothe aperture and contact pads 460 arranged on the surface of theperiphery of the substrate 456 are soldered to mating contacts 462 onprinted circuit board along the periphery of the aperture 458.

Techniques for over molding electronic components on one side of asubstrate are known. In one example, electronic devices mounted on oneside of a printed circuit board assembly are over-molded withencapsulant and the other side of the printed circuit board assembly,which is not over-molded, comprises a ball grid or a land grid array ofelectrical contacts. FIG. 13 illustrates a ball grid array package ofthe kind shown in a datasheet for a “Full Function Synchronous BuckPower Block”, model iP1001, published by International Rectifier, ElSegundo, Calif., USA. In the figure, power conversion circuitry (notshown) consists of components mounted on top of a circuit board. Thecomponents and the board are over-molded with encapsulant to form apackaged device 232. A ball grid array of contacts (e.g., contacts 233in FIG. 13) is arranged along the bottom surface of the device. Inapplication, the ball grid array of contacts is soldered to matingcontact pads or runs (e.g., contact pads 235) on the surface of aprinted circuit board (“PCB”) 239. The package architecture exemplifiedabove, sometimes referred to as “System In a Package” (SIP), providessome of the electrical, mechanical and thermal managementcharacteristics required of high power density and high current densityconverters. However, the SIP architecture is incompatible with two-sidedcircuit board assembly including transformer core structures protrudingfrom both sides of the circuit board, as described in the FactorizedApplication ibid. Furthermore, the SIP package provides limitedmechanical and thermal management flexibility.

Intel Corporation, Santa Clara, Calif., USA, manufacturesmicroprocessors which are packaged in a package, called a Micro-FCPGApackage, which comprises a component over molded on one side of asubstrate and a pin-grid-array and exposed capacitors on the other sideof a substrate.

Saxelby, Jr., et al, U.S. Pat. No. 5,728,600, “Circuit EncapsulationProcess” and Saxelby, Jr., et al, U.S. Pat. No. 6,403,009, “CircuitEncapsulation” (both assigned to the same assignee as this applicationand both incorporated in their entirety by reference) describe ways ofover-molding both sides of a printed circuit board assembly whileleaving opposing regions on both sides of the printed circuit board freeof encapsulant. This is useful for exposing a row of contacts thatextend along an edge of the printed circuit board on both sides of theboard.

SUMMARY

In general, in one aspect, a method for encapsulating two sides of asubstrate includes providing a mold including a first mold sectionhaving a first cavity for encapsulating a first region of a firstsurface of the substrate and a second mold section having a secondcavity for encapsulating a second region of a second surface of thesubstrate. A fill conduit for introducing encapsulating material intothe first cavity is provided at a first end of the mold. A channelhaving an opening in the first cavity at an end opposite the first endfor allowing encapsulating material to flow from the first cavity intothe second cavity is also provided.

In general, in another aspect, a method for encapsulating two sides of asubstrate includes closing a mold on the substrate. A first mold sectionhas a first cavity for encapsulating a first region of a first surfaceof the substrate and a second mold section has a second cavity forencapsulating a second region of a second surface of the substrate. Asealing force for forcing the substrate against the second mold sectionto seal the second cavity is created by injecting encapsulating materialinto the first cavity.

Implementations of the general methods may include one or more of thefollowing features. A step-over cavity may be provided in the secondmold section outside of the second cavity for accommodating featuresprotruding from the second surface of the substrate. The mold may beclosed on the substrate, encapsulating material may be forced underpressure through the fill conduit, and the second cavity may be filledwith encapsulating material conducted through the channel from the firstcavity. The second region may be smaller than the first region. Thechannel may include an aperture through the substrate. Encapsulatingmaterial may be conducted through a channel from the first cavity tofill the second cavity. The encapsulating material may be injected intoa first end of the first cavity and conducted from the first cavity froma second end opposite from the first end.

In general, in another aspect, an apparatus includes a thermal extenderhaving a first surface and second surface separated by a thickness. Afirst surface area on the first surface is adapted to mate with a heatdissipating power converter and a second surface area on the secondsurface is adapted to mount on an external circuit board. A plurality ofinterface conductors have a first end on the first surface for matingwith contacts on the heat dissipating power converter and a second endfor mating with conductive regions on the external circuit board. A heatsink is thermally coupled to the first surface area for dissipating heatgenerated by the power component.

Implementations of the general apparatus may include one or more of thefollowing features. The heat sink may be surface mounted to the firstsurface. A thermally conductive layer may extend from the first surfacearea to the heat sink and the heat sink may include an extension of thethermally conductive layer. The thermal extender may comprise athermally conductive molding and the heat sink may comprise heat sinkelements. The thermal extender may include an aperture for accepting alower portion of the heat dissipating power converter and the firstsurface area may be adapted for surface mounting of the converter to theextender.

In general, in another aspect, a method to cool a power converterincludes thermally coupling the power converter to an external circuitboard and thermally coupling the external circuit board to a heat sinkwith a thermal resistance between the power converter and the heat sinkof less than 80 C/Watt per cm of the power converter perimeter.

In general, in another aspect, a method for cooling a heat dissipatingpower converter includes providing a thermal extender having a firstsurface and second surface separated by a thickness and including aplurality of interface conductors having a first end on the firstsurface and a second end on the second surface; thermally coupling thepower converter to the first surface of a thermal extender and matingcontacts on the power converter to the first ends of the interfaceconductors; thermally coupling the second surface of the thermalextender to an external circuit board and mating conductive regions onthe external circuit board to the second ends of the interfaceconductors; and thermally coupling a heat sink to the first surface fordissipating heat generated by the power converter.

Implementations of the general methods may include one or more of thefollowing features. The thermal resistance may be less than 40 C/Wattper cm of the power converter perimeter.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a prior art power conversion apparatus.

FIGS. 2A-2C show, respectively, an exploded perspective view, a cutawayperspective view and a side view of another prior art power conversionapparatus.

FIGS. 3A and 3B show, respectively, an exploded perspective view and acutaway perspective view of a power converter apparatus according to theinvention.

FIGS. 4A and 4B show perspective views of power converters according tothe invention.

FIGS. 5A and 5B show top and bottom phantom perspective views of a powerconverter of the kind shown in FIG. 4B.

FIGS. 6A and 6B show, respectively, an exploded perspective view and aperspective view of a power converter apparatus comprising aninterconnect extender according to the invention.

FIG. 7 shows a cross-sectional view of a portion of a power converterapparatus comprising an interconnect extender according to theinvention.

FIG. 8 shows an exploded cross sectional view of a power converterapparatus comprising an interconnect extender and a pin and socketarrangement.

FIGS. 9A and 9B show cross-sectional views of different embodiments ofthe power converter apparatus of FIG. 6.

FIG. 10 is a partial cross sectional view of a power converter apparatusshowing heat flow between components in a power converter and a circuitboard.

FIGS. 11A and 11B show, respectively, perspective views of powerconverter apparatus that is connected to a circuit board and cooled by aheat sink attached to the circuit board. FIGS. 12 and 12B show anexploded perspective view and perspective view of power converterapparatus comprising a thermal extender.

FIG. 13 shows an exploded perspective view of a prior art packagecomprising a ball grid array.

FIGS. 14A and 14B show, respectively, cross-sections of a mold withoutand with a step-over cavity for pre-attached interface contacts.

FIGS. 15A and 15B show, respectively, perspective and side views of apower converter mounted to a circuit board via a vertical interconnectextender and card-edge connector.

FIGS. 16A, 16B, and 16D show, respectively, side and perspective viewsof a power converter mounted to a circuit board using a verticalinterconnect extender.

FIG. 16C shows a perspective view of a vertical interconnect extender.

FIG. 16E shows contacts for use with a vertical interconnect extender.

FIGS. 17A, 17B and 17C illustrate a molding apparatus and method.

FIG. 18 shows an exploded perspective view of a prior art integratedpassive network mounted over an aperture in a circuit board.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIGS. 3A and 3B show, respectively, an exploded perspective view and acutaway perspective view of a power converter apparatus 30. Theapparatus comprises a power converter 32 and a circuit board assembly39. A perspective view of the power converter 32, viewed from below, isshown in FIGS. 4A and 4B. Referring to FIGS. 3 and 4, the powerconverter comprises an upper portion 36 and a lower portion 31 which aregenerally of the form of a rectangular parallelepiped. The upper portionhas a generally rectangular upper surface 35 and an overhang surface 42.The lower portion has a generally rectangular bottom surface 34. Thesides of the lower portion 31 (e.g., sides 46 a, 46 b) are shorter inlength than the sides of the upper portion 36 (e.g., sides 44 a, 44 b)allowing the upper portion 36 to overhang the lower portion 31 andexposing the overhang surface 42. The lower portion may be centered onthe upper portion as shown in FIGS. 4A and 4B establishing symmetricalregions of overhang (e.g., overhang regions 42 a, 42 b, 42 c, 42 d) onoverhang surface 42. Power and signal input-output contact connectionsto the power converter may be made via interface contacts 33 (e.g.,solderable contacts, such as a ball-grid-array (“BGA”) or a land gridarray (“LGA”)) formed on the overhang surface 42, along regions 42 a-d.The interface contacts 33 may be arranged along the entire periphery(e.g. regions 42 a-d as shown in FIG. 4A) or a portion of the periphery(e.g. the 2 long regions 42 b, 42 d as shown in FIG. 4B).

The power converter 32 may be mounted to the circuit board 39 in a“through the board” configuration as shown in FIGS. 3A and 3B. Anaperture 45 is provided in the circuit board 39 to allow the lowerportion 34 to protrude into the aperture. Preferably, the aperture 45matches the periphery of the lower portion 34 in size and shape.Conductive runs (e.g. runs 37, 38) are provided on the circuit board 39for mating with the interface contacts 33. The apparatus 30 is assembledby placing the lower portion 34 of the power converter 32 into theaperture 45 in the circuit board 39 and soldering the interface contacts33 to the conductive runs (e.g. runs 37, 38).

Internal details of an embodiment of the converter 32 of FIG. 4B areshown in phantom view in FIGS. 5A and 5B. The converter may be a sineamplitude converter of the kind described in connection with FIGS. 40Aand 40B in the Factorized Application ibid. Reference numerals used inFIGS. 5A and 5B herein, generally correspond to the reference numeralsused in FIGS. 40A and 40B of the Factorized Application. FIGS. 5A and 5Bshow top and bottom views of the circuit board 442. As illustrated inFIGS. 5A and 5B, components are mounted to both surfaces of the circuitboard 442. Power dissipating components such as MOSFET power switches448, synchronous rectifiers 453, 455 and/or other components (“upperelectrical components”) may be arranged on the top surface of thecircuit board 442. Input capacitors 456, resonant capacitors 430, outputfilter capacitors 458, 460 and/or other components (“lower electricalcomponents”) may be arranged on the bottom surface of the circuit board442. Magnetic components, such as isolation transformer core sets 422 a,422 b, may extend over the top and bottom surface of the circuit board.In applications where heat is primarily removed by forced airflow or byan external heat sink coupled to the top surface 35, cooling of powerdissipating components, such as MOSFET power switches 448, is enhancedby placing the devices on the top surface of the circuit board 442, inclose proximity to the large top surface 35. On the other hand, energystorage elements, such as ceramic capacitors (e.g. 456, 458) do notgenerate as significant an amount of heat, but are thicker and thusrequire greater headroom. Such components are preferably located on thebottom surface of the circuit board where greater package headroom isprovided. In a preferred embodiment, the upper portion accommodatescomponents with a height of less than 1.2 mm and the lower portionaccommodates components with a height of less than 2.0 mm.

The top and bottom surfaces of the circuit board may be encapsulated inthermally conductive epoxy (e.g., EME-LK4-2, manufactured by SumitomoBakelite Co. Ltd.) to form the upper and lower portions, e.g., theparallelepiped upper and lower portions 36, 31 shown in FIGS. 3 and 4.The outer surfaces of the encapsulating material may form the outersurfaces of upper portion 36 and lower portion 31 including top surface35 (as indicated by dashed lines 336) and bottom surface 34 (asindicated by dashed lines 331) shown in FIGS. 5A and 5B. With allowancewithin the upper portion for components with a height of less than 1.2mm, the average thermal resistance between the top surface of thecircuit board and the external top surface of a package encapsulatedusing the above referenced LK4-2 is approximately 1.6 C/Watt per squareinch of the top surface of the circuit board. A portion of the circuitboard 442 corresponding to the overhang surface 42 (regions 42 a-42 d)is left un-encapsulated to expose contact pads (e.g., contact pads 63 inFIG. 5B) to which interface contacts 33 (e.g., solder balls in a BGA, orother contacts) may be attached. The completed overhang packagearchitecture, e.g., the dual rectangular-parallelepiped packageillustrated in FIGS. 3, 4, and 5, provides electrical interconnect,mechanical mounting and thermal cooling advantages over existing packagearchitectures lacking the overhang, e.g. a singlerectangular-parallelepiped surface mount package.

In general, electrically, the package interconnect resistance andinductance are optimized by proximity of a multiplicity of interconnectelements to the components contained in the body of the package.Mechanically, the package optimizes the power density of the powerconverter by optimal allocation of relatively thinner semiconductorpower devices in the upper portion of the package and relatively thickerenergy storage components in the lower portion of the package. Theeffective power density of a system incorporating a power converter withthis package is further optimized by the ability to insert the lowerportion of the package within an aperture of an external circuit boardwhile supporting the package over the circuit board with its overhang.Mechanical mounting flexibility is also provided by the ability tocouple the package through an interposer or extender element to allowsurface mounting over an external board, with the package body beingparallel to the surface of the board. By using the volume under thepackage overhang, an interposer may be added without substantiallyraising the height of the overall package, both for low profile surfacemounting and for vertical mounting with minimized footprint. Thermalmanagement of the heat generated within the package is facilitated bythe ability to extract heat by conduction or convection from the topsurface of the package, with low thermal resistance to the semiconductorjunctions contained within the upper portion of the package. Thermalmanagement is also effectively provided by conduction into an externalcircuit board or interposer element, with low thermal resistance betweenthe semiconductor junctions thermally coupled to the top surface of thecircuit board and the interface contacts to the external circuit board,the removal of heat being facilitated by lateral conduction along thecircuit board within the package to its interface contacts, orterminals, under the package overhang.

Interface contacts 33 (e.g., solder balls or brass balls in a BGA) maybe applied to the contact pads 63 before or after encapsulation orover-molding. Individual solder balls may be applied to the contact padsto form a BGA using standard adhesive tape carrier, robot placement, orstencil methods. Because it is difficult in practice to operate amolding process without some resin bleed or flash occurring at theparting line of the mold and a parting line exists between the exposedoverhang 42 and the encapsulated lower portion 31, resin bleed mayadhere to the contact pads 63 preventing proper attachment of theinterface contacts 33 without an additional process step to clean offthe bleeding which would otherwise cause need for a dedicated cleaningstep. With pre-attached contacts, resin that bleeds around the base ofthe solder spheres will not interfere with the integrity of the BGA aslong as the thickness of the bleeding film is substantially less thanthe diameter of the balls. The encapsulation-mold cavity may be designedto “step over” interface contacts 33 (e.g., solder balls in a BGA orother contacts) attached to contact pads 63 before the molding process.

Referring to FIG. 14A, a cross-section of a mold 510 includes moldsection 501 with cavity 502 which closes on the bottom surface ofcircuit board 442 and mold section 504 with mold section 503 whichcloses on the top surface of circuit board 442 to encapsulate,respectively, the lower portion 31 and upper portion 36. The exposedoverhang surface 42 of circuit board 442 mates with mold section 501 inthe area indicated by reference designation 507 in FIG. 14A. The mold520 illustrated in FIG. 14B includes a step-over cavity 506 in moldsection 505 to provide clearance for pre-attached solder spheres 33 asshown. The step-over cavity may be used with other types of interfacecontacts also. The overhang surfaces in regions between previouslyattached interface contacts 33 (e.g., LGA or BGA contacts) may beencapsulated in alternative embodiments.

The packaging architecture described with reference to FIGS. 3, 4, and 5is not limited to packages having upper and lower portions that aregenerally parallelepiped in shape. The above described encapsulationmethod of forming the upper and lower portions is particularly wellsuited to fashioning the upper and lower portions in virtually any shape(e.g., polyhedral, cylindrical, etc.). In the case of a generallyparallelepiped upper region, the package may advantageously includeinterface contacts on all or a portion of its overhang surfaces.Specifically, the overhang surface may be formed on a multiplicity ofsides (e.g., 2, 3, etc.) of the parallelepiped. In embodiments where theupper region is not generally parallelepiped in shape, the interfacecontacts can be arranged anywhere along the overhang region.

In applications where it is not possible or desirable to use “throughthe board” mounting, the overhang package may be mounted above thesurface of the circuit board in an “on board” configuration using aninterposer such as an interconnect extender. FIGS. 6A and 6B show,respectively, an exploded perspective view and a perspective view of apower converter apparatus 54 comprising a power converter 32 packaged inan overhang package of the kind described above, a circuit board 39, andan interconnect extender 50. Interconnect extender 50 has an aperture 52for accommodating the lower portion 31 of the overhang package andextender contacts 53, which connect contacts 33 on the converter 32 toconductive runs 37, 38 on the circuit board 39. The height, H1, of theextender 50 is preferably greater than or equal to the height, H2, ofthe lower portion 31 of the power converter under all reasonabletolerance conditions to provide the required clearance between thebottom surface 34 and the circuit board 39.

The interconnect extender 50 may also be used to provide a variety ofalternative lead terminations to the power converter. Referring to FIG.7, a cross-sectional view of a portion of a power converter assemblycomprising a packaged power converter 32 of the kind described above, acircuit board 39 with a conductive run 38 and an interconnect extender50 is shown. The end 55 of the extender contact 53 that attaches to theconductive run 38 on the circuit board 39 is in the form of a “J” lead.Alternatively, other lead terminations, such a through-hole leads orgull-wing terminations may be used.

An interconnect extender may also be used to create a plug-and-socketconnection between a packaged converter, of the kind described above,and a circuit board assembly. Referring to FIG. 8, an explodedcross-sectional view of a power converter apparatus using aninterconnect extender 50 with extender pins 60 is shown. The top ends ofthe extender pins (not shown) are soldered to the interface contacts 33on the power converter 32 in a manner similar to that shown in FIG. 7.The lower ends of the pins 60 extend from the bottom of the interconnectextender 50, and may be inserted into sockets 61 that are themselvessoldered to runs 38 on circuit board 39. Although FIG. 8 shows the powerconverter 32 and extender 50 assembly as an “on-board” mountingarrangement, the plug-and-socket technique may also be used in “throughthe board” applications by reducing the height H1 of the extender 50(FIG. 6A) to less than the height H2 (FIG. 6A) of the lower portion 31allowing the lower portion to protrude into the plane of the circuitboard.

The interconnect extender may also be used in “through the board”configurations to vary the extent to which the power converter bodyprotrudes down into the mounting surface e.g., the depth to which thelower portion 31 extends into or through the circuit board. For example,FIGS. 9A and 9B show cross-sectional views of portions of two powerconverter assemblies: the assembly in FIG. 9A uses an extender 57 awhich is shorter in height than the extender 57 b of the assembly ofFIG. 9B. As a result, the distance from the bottom surface 34 of thepower converter 32 to the bottom surface of the circuit board 59 issmaller in FIG. 9B than it is in FIG. 9A. Adjusting the relativeextensions of the top and bottom surfaces of the power converter 32relative to the surfaces of the circuit board 39 may be used to adjustrelative airflow across the top surface and bottom surface of theconverter.

In embodiments in which a plurality of interface contacts 33 of a fixedsize is used (e.g., interface contacts of the ball-grid array type), thecurrent carrying capacity of an individual contact may be relativelylow. In such cases, a plurality of contacts can be used together tocreate a high current interface. Input terminals may be located on oneside of the package and output terminals may be located on the oppositeside. With the package having a rectangular shape, including a long sideand a short side, it is desirable to locate the input and outputcontacts along the long sides. This increases the proximity between theinterface contacts and the innards of the package to which and fromwhich currents need to flow and it reduces the thermal resistancebetween semiconductor junctions within the package and its interfacecontacts.

Some benefits of the power conversion package and apparatus describedabove are: small size; high interface contact density owing to theavailability of the periphery of the package in the region of theoverhangs 42 for placement of interface contacts; exposure of two largesurfaces e.g. top and bottom surfaces 34, 35, for heat removal andcooling, the two surfaces representing a significant percentage of thetotal package surface area; generally high equivalent power densityowing to a portion of the package being coextensive with the thicknessof the circuit board 39; short, direct, interface connections betweenthe converter and the circuit board, resulting in relatively lowresistance and inductance in the electrical connections; and flexibilityand ease of assembly in both “through the board” and “on board”applications.

Heat removal from the power converter package, specifically from powerdissipating components contained within the package, may be providedusing the printed circuit board 442 and the interface contacts 33 of aconverter embodiment of the kind shown in FIG. 5. Referring to FIG. 10,a power converter 32, of the kind shown in FIG. 5, is shown mounted to acircuit board 39 in cross-section. The converter 32 includes amultilayer PCB 442 and circuit components (e.g., components 447, 449mounted on, respectively, the top and bottom surfaces of multilayer PCB442) encapsulated in thermally conductive epoxy 451. Features in FIG.10—in particular, the relative thicknesses of PCB 442 and circuit board39—are not drawn to scale. The multilayer board 442 may include severalalternating layers of conductive runs (e.g., conductive runs 65 athrough 65 f) and insulating layers (e.g., insulating layers 67 athrough 67 e), which may comprise, for example, impregnated fiberglasssubstrates or ceramic substrates. The conductive runs 65 formconnections between circuit components mounted on the top and bottomsurfaces of the PCB 442 and between converter circuitry and interfacecontacts 33 shown in FIG. 10 as solder connection 62 representing aportion of a reflowed BGA. Solder connection 62 connects conductive run67 f, on the bottom of multilayer PCB 442, to a conductive run 66 oncircuit board 39.

In FIG. 10, a heat-dissipating component 447 is mounted onto conductiverun 65 a on the top surface of multilayer printed circuit board 442. Asindicated by the arrows 72 in the Figure, heat flows both vertically andhorizontally from the component 447 into the multilayer board 442. Ametallic plated-through via 64 participates in conducting the heat downthrough solder connection 62 into conductive run 66 on circuit board 39.

The effectiveness of the heat removal technique illustrated in FIG. 10may be illustrated with the following example in which a power convertercomprising heat-dissipating components is assumed to dissipate heatapproximately uniformly over the top surface of a multilayer board.Assuming that the multilayer board measures 0.82 inch (21 mm)×1.26 inch(32 mm) along the edges, consists of fourteen layers of 2-ounce copper(approximately 0.085 inch (2.16 mm) thick) etched into conductive runsand separated by thirteen 0.004 inch (0.16 mm) thick relatively highthermal conductivity (2.5 Watt/m-C) insulating layers, the “effectivethermal resistance” between the top surface of the multilayer board andthe two long edges of the package will be of the order of 1 C/Watt.Dielectric substrate (pre-preg) materials incorporating a thermallyconductive filler, such as ceramic, may be used to fabricate thermallyconductive printed circuit boards. For example, T-Lam available fromThermagon, Inc., 4707 Detroit Avenue, Cleveland, Ohio 44102-2216, andThermal-Clad available from The Bergquist Company, 18390 W. 78th Street,Chanhassen, Minn. 55317 may be used. Assume that a ball-grid arrayconsisting of 108 solder balls 0.75 mm in diameter is used for interfacecontacts 33 on the overhang surface and the solder balls are reflowed toform solder joints between conductive runs on the bottom surface of themultilayer board (e.g., run 65 f, FIG. 10) and conductive runs on acircuit board (e.g., run 66, FIG. 10). Each resulting solder joint(e.g., solder joint 62, FIG. 10) will have a thermal resistance ofapproximately 20 C/Watt. The BGA (consisting of 108 such solder balls)will therefore contribute a thermal resistance of approximately 0.2C/Watt. The total “effective thermal resistance” between the top layerof the multilayer board 442 and the circuit board 39 will beapproximately 1.2 Watt/C (or, 16 C/Watt per cm of periphery, given aperiphery of 13.5 cm). With a thermal load of less than 10 Watts, the“average” temperature rise between the top surface of the multilayerboard 442 and the solder joint 62 on the run 66 on the circuit board 39can be kept below 12 C allowing for effective and low cost thermalmanagement of the converter.

Heat conducted from the converter 32 into the circuit board 39 may beexchanged with the surrounding system in a variety of ways. Referring toFIG. 11A, the surface area of the circuit board may be sufficient, whencombined with the surface area of the top and/or bottom surfaces of theconverter 32, to exchange heat with the surrounding air as a means ofcooling the converter in some applications. The wavy arrows in FIG. 11Arepresent the flow of heat from the power converter 32, through thesolder joints (e.g., solder joint 62), into the circuit board 39, andinto the surrounding environment. As shown, the spreading of heat may beenhanced using highly thermally conductive metal runs 69 on the circuitboard 39. Heat is removed from the surfaces of the circuit board 39,runs 69, and from the surfaces of the converter 32, by free or forcedconvection. Although only the top surfaces of the converter 32 andcircuit board 39 are shown in FIG. 11A, the bottom sides of theconverter 32 (in through the board applications) and the bottom side ofthe circuit board 39, and conductive runs thereon, may contribute to thetransfer of heat into the environment.

Alternatively, one or more small heat sinks may be deployed along thesides of the converter package. Referring to FIG. 11B, two heat sinks 71are shown, for example, surface mounted (e.g., by use of solder orthermally conductive adhesive) to conductive runs 69 on circuit board39. The heat sinks 71 may have the same height or be taller or shorterthan the converter 32 depending upon the application. Using heat sinks,which provide substantial surface area for heat convection whileoccupying relatively little surface area on the circuit board 39,adjacent to the converter package is more space efficient in comparisonto the technique of FIG. 11A. The adjacent heat sinks reduce thedistance over which the heat flows in the circuit board 39 furtherimproving heat transfer between the converter and the environment overthe embodiment of FIG. 11A. Although shown mounted to the top surface,heat sinks 71 may be mounted to the bottom surface of the circuit board39 instead of or in addition to heat sinks on the top of the circuitboard. Either or both of the top and bottom surfaces of the converter 32may aid in heat removal. Additionally, heat sinks may be attached to anyor all of the top, bottom, and side surfaces of the power converter tofurther improve heat transfer to the environment. Free or forcedconvection may be used.

Furthermore, having conducted heat from the power converter to theexternal circuit board as exemplified in FIG. 10, heat may be removedfrom the external circuit board by conduction through heat sinksconnected to other parts of the system. For example, a heat sink may beconnected between the top surface of the external circuit board and acold plate located above the top surface of the circuit board.Alternatively, a heat sink may be connected between the bottom surfaceof the external circuit board and a cold plate located below the bottomsurface of the circuit board.

The thermal management techniques discussed above in connection withFIGS. 10 and 11 may be adapted to applications in which an interconnectextender is used. Referring to FIG. 12A, an interconnect extender 250may be adapted to provide, in addition to electrical interconnections toan external circuit board, low thermal resistance to a heat sink. Such athermal extender may incorporate a heat sink to remove heat from thepower converter and exchange it with the surrounding air. A thermalextender reduces the total thermal resistance between the powerconverter and the system surrounding it. One or more heat sinks 71 maybe surface mounted to runs 69 on the top surface of the interconnectextender 250. The power converter 32 may be connected to theinterconnect extender 250 using a BGA or LGA. The interconnect extender250 is preferably connected to the top surface of circuit board 39 usinga BGA, LGA, or alternative lead terminations, as previously described.The heat sinks in FIG. 12A will operate essentially as described abovewith respect to FIG. 11B, except that the heat is conducted from thepower converter 32, through the solder joints (not shown), alongconductive runs 69 on the extender (rather than on the circuit board),out through the heat sinks 71 into the environment. The interconnectextender 250 may be constructed using materials similar to thosedescribed above in connection with circuit board 442 in FIG. 10.

An alternative thermal extender with heat sink elements is shown in FIG.12B. Referring to FIG. 12B, the thermal extender 251 may be molded orotherwise constructed using a high thermal conductivity material (suchas CoolPoly from CoolPolymers Inc., 333 Strawberry Field Rd., Warwick,R.I. 02886 USA). Such thermally conductive plastics (e.g., CoolPoly D, athermally conductive liquid crystalline polymer) achieve thermalconductivities of the order of 15 W/mK, allowing effective heat transferwith relatively low thermal gradients. While cooling effectiveness maybe comparable to metals, thermally conductive plastics are lightweight,moldable and insulating. In these embodiments, heat sink elements 72 maybe molded as extensions of the base of the thermal extender 251, insteadof being discrete surface mount heat sinks attached to the surface ofthe extender, as discussed above in connection with FIG. 12A. With thebase of the thermal extender being a dielectric material, electricalinterconnect elements may be incorporated into the extender by insertingmetal pins, such as brass pins, within holes or other features moldedwithin the base of the extender. The fabrication of the extender,including insertion of the interconnect elements, may be realized usingprocesses similar to those utilized in fabricating electricalconnectors. By providing thermal extenders (e.g., extenders 250 of FIGS.12A and 251 of FIG. 12B) which use a variety of different heat sinkconfigurations 71, a supply of otherwise identical converters 32 may beadapted to operate in different thermal environments by appropriateselection of the extender.

Advantages of the thermal management techniques shown in FIGS. 10, 11and 12 include allowing for a very low profile system, including thecircuit board; avoiding the need to attach one or more heat sinks to thetop and bottom surfaces of the converter 32 while still effectivelyusing these surfaces for heat exchange; and flexibility in rapidlyadapting a supply of converters to different circuit boards andenvironments.

It may be desirable in some applications to mount the power converter 32vertically. Several problems with vertically mounting a power converterto a printed circuit board include providing mechanical stability duringsoldering (preventing it from falling over or shifting positions) andfor the life of the product (shock and vibration endurance), andmaintaining the integrity of the solder connections between the powerconverter 32 and a vertical mount interface extender while the verticalmount assembly is being soldered to a customer PC board.

One vertical mounting technique which solves these problems is shown inFIGS. 15A and 15B. Referring to FIGS. 15A and 15B, a power converter 32is mounted to an interconnect extender 253, which may be a printedcircuit board. The power converter 32 is shown mounted to the extender253 in a through the board configuration similar to that shown in FIGS.3A and 3B. The bottom surface 34 of the lower portion of the converter32 is shown extending through an aperture in the interconnect extender253 in FIG. 15A. The upper surface 35 of the converter can be seen inFIG. 15B. Interconnect extender 253 includes contacts e.g., edge-fingercontacts 260, 261, arranged along an edge of the extender on one or bothsides of the extender for mating with a connector 262, e.g., a card edgeconnector. The contacts 260, 261 may be connected via conductive runs(not shown) to interface contacts (e.g. interface contacts 33 in FIG.3A) on the power converter 32. The card edge connector 262 may besoldered to conductive runs on circuit board 39 using surface mount orother techniques.

Although the card-edge connector technique of FIGS. 15A and 15B providesthe necessary mechanical foundation and avoids problems encountered withsoldering, it may be a relatively low-performance vertical mountsolution from a number of points of view, such as thermal and electricalperformance. The interconnect extender 253 of FIGS. 15A and 15B mayfurther comprise heat sinks such as shown in FIGS. 12A and 12B.

Alternatively, a vertical mounting interconnect extender adapted to besoldered to a PCB may be used. Referring to FIG. 16A through 16E, avertical interconnect extender 254 for supporting a power converter 32is shown. Referring to FIG. 16C, the extender 254 includes a PCB 252having an aperture 255 for accommodating the lower portion of the powerconverter 32 in a “through the board” mounting arrangement such as theone shown in FIGS. 3A, 3B. The bottom surface 34 of the converter 32 isshown extending through the interconnect extender 254 in FIGS. 16A and16D. The interconnect extender 254 may use a PCB 252 similar to theextender 253 of FIG. 15, except that instead of mating with a card-edgeconnector, the vertical-mounting extender 254 includes leads 263 (shownin greater detail in FIG. 16E) e.g., NAS/Interplex “Dual RowInterconnect” formed for surface mount assembly to circuit board 39. ThePCB 252 includes conductive runs (not shown) to connect the interfacecontacts on the power converter to the leads 263. The leads 263 may besoldered and adhesively bonded to the circuit board 252 for stabilityduring subsequent soldering to circuit board 39. Thermal conductivegussets 270 may be provided to add mechanical stability and to decreasethe thermal resistance between the extender and the circuit board 39.The gussets 270 may be mechanically attached (or soldered and adhesivelybonded) to the circuit board 252 of extender 254 and soldered to pads(e.g. pads 271 in FIG. 16E) on circuit board 39. The gussets 270 andleads 263 help conduct heat away from the converter 32 and into thecircuit board 39 similar to the techniques discussed above in connectionwith FIGS. 10-11. Heat sinks may be added to the extender 254, one ormore surfaces of the power converter 32, or to the circuit board 39 asdiscussed above. A spring clip (not shown) or adhesive may be added tosecure the power converter 32 to the above described interconnectextenders to provide mechanical stability during solder operations.Solder, having a melting point higher than that of the solder used toattach the vertical extender to the circuit board 39, may be used toattach the power converter 32, leads 263, and gussets 270 to the circuitboard 252 to avoid problems during the later solder operations. Thegussets may also be provided with features such as pins or fingers wherethey attach to the circuit board 39 or to PCB 252 to prevent slidingduring assembly and soldering operations.

As described above, the upper and lower portions (e.g., the generallyparallelepiped upper and lower portions 36, 31 shown in FIGS. 3A, 3B, 4Aand 4B) may be formed by encapsulating the top and bottom surfaces of acircuit board. FIGS. 17A through 17C illustrate how the top and bottomsurfaces of a printed circuit board may be encapsulated in a mold whileleaving a predetermined area on one side of the board free ofencapsulating material. The cross-sectional views of FIGS. 17A and 17Bare taken at the same first location; the cross-sectional view of FIG.17C is taken at a second location which is at a right angle to the crosssection of FIGS. 17A and 17B.

Referring to FIGS. 17A and 17B, upper mold section 401 forms an uppercavity 404 above the top surface 406 of printed circuit board 442 and isin contact with the top surface 406 of the printed circuit board 442 atupper contact regions 413 a, 415 a. Likewise, lower mold section 402forms a lower cavity 403 below the bottom surface 405 of printed circuitboard 442 and is in contact with the bottom surface 405 of the printedcircuit board 442 at lower contact regions 409 a, 411 a. Referring tothe cross-section of FIG. 17C, the upper mold section 401 comes incontact with the top surface 406 of printed circuit board 442 at uppercontact regions 413 b, 415 b and the lower mold section 402 comes incontact with the bottom surface 405 of the printed circuit board 442 atlower contact regions 409 b, 411 b. Additionally, the lower mold section402 may optionally come in contact with the bottom surface 405 of theprinted circuit board 442 at lower contact regions 409 c, 411 c.Step-over cavities 417 a, 417 b in the lower mold section 402 provideclearance for BGA solder balls 33 (or other interface contacts) on thebottom surface 405 of printed circuit board 442. The upper mold cavity404 is used to form an encapsulated upper portion (corresponding, e.g.,to upper portion 36 in FIG. 4B) on the top surface 406 of the printedcircuit board 442. The lower mold cavity 403 is used to form anencapsulated lower portion (corresponding, e.g., to lower portion 31 inFIG. 4B) on the bottom surface 405 of the printed circuit board 442.Since the regions 423 a, 423 b on the bottom surface 405 of the printedcircuit board 442 are kept free of encapsulant, the surface area that iswithin the periphery defined by lower contact regions 409 a, 409 b, 411a, 411 b on the bottom surface 405 of the printed circuit board 442 (the“lower region area”) is smaller than the surface area that is within theperiphery defined by upper contact regions 413 a, 413 b, 415 a, 415 b onthe top surface 406 of the printed circuit board (the “upper regionarea”). As described below, the arrangement of the mold and the printedcircuit board shown in FIGS. 17A-17C enables molding of the upper andlower portions while keeping contact regions 423 a, 423 b (under thestep-over cavities 417 a, 417 b on the bottom surface of the printedcircuit board 442) essentially free of encapsulant.

Referring to FIGS. 17A and 17B, movement of the plunger 407 forcesliquefied encapsulating material 420 into the upper cavity 404 underpressure in the direction indicated by the arrows. As the flow ofencapsulating material 420 fills the upper cavity 404, the pressureexerted by the encapsulating material on the top surface 406 of theprinted circuit board creates a net downward sealing force which pushesthe bottom surface 405 of the board against the lower mold section 402along the periphery defined by the lower contact regions 409 a, 409 b,411 a, 411 b. The net downward sealing force prevents encapsulatingmaterial from flowing into contact regions 423 a, 423 b. After the uppercavity 404 has filled with encapsulating material 420, the encapsulatingmaterial will flow into the lower cavity 403 via the conduit 419 formedin the printed circuit board 442 (FIG. 17B). Because the “lower regionarea” is smaller than the “upper region area,” a net downward sealingforce is maintained throughout the filling of the lower cavity 403 withencapsulating material 420, as illustrated by the solid arrows in FIG.17C.

In certain open-frame applications, encapsulation within a package asdescribed above may be unnecessary and there may be a benefit toexposing the components of the power converter to an external airflow toprovide direct cooling of components. The open-frame applications mayalso benefit from various aspects of the invention described above,including those stemming from the use of interface contacts arrangedwithin an overhang region on the bottom surface of the power convertercircuit board. In particular, a BGA for making electrical connections tothe power converter may be arranged within the overhang region and thepower converter may be mounted by the BGA connection to an externalcircuit board. The circuitry on the bottom of the power converter mayextend into an aperture in the external circuit board, to reduce theoverall height the assembly while minimizing electrical and thermalinterconnect impedances. The power converter of FIGS. 5 and 10 may beassembled and mounted without the package or encapsulation elements(shown as an outline in FIG. 5) for use in open-frame applications. Theresulting open-frame power converter may be mounted to the externalcircuit board in the same manner as shown in FIG. 10 with the components(e.g. component 449) sitting in the aperture in the external circuitboard (however, epoxy 451 would be omitted). The package outline (e.g.in FIGS. 5A, 5B) and encapsulation elements (e.g. in FIG. 10) shown inthe figures may be viewed as an outline of the power converter inopen-frame applications that omit the package or encapsulation. Theopen-frame power converter may also be mounted to an external circuitboard using an interconnect extender located under the overhang regionas shown in FIGS. 6A, 6B, 9A and 9B.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method for encapsulating two sides of a substrate comprising: providing a mold including a first mold section having a first cavity for encapsulating a first region of a first surface of the substrate and a second mold section having a second cavity for encapsulating a second region of a second surface of the substrate; sealing the first mold section against the first surface of the substrate around a periphery of the first region; sealing the second mold section against the second surface of the substrate around a periphery of the second region; providing a fill conduit for introducing encapsulating material into the first cavity at a first end of the mold; and providing a channel having an opening in the first cavity for allowing encapsulating material to flow from the first cavity into the second cavity.
 2. The method of claim 1 further comprising: forcing encapsulating material under pressure through the fill conduit; filling the second cavity with encapsulating material conducted through the channel from the first cavity.
 3. The method of claim 1 wherein the fill conduit is provided at a first end of the mold and the channel opening in the first cavity is provided at an end opposite the first end.
 4. A method for encapsulating two sides of a substrate comprising: closing a mold on the substrate, including a first mold section having a first cavity for encapsulating a first region of a first surface of the substrate and a second mold section having a second cavity for encapsulating a second region of a second surface of the substrate; creating a sealing force for forcing the substrate against the second mold section to seal the second cavity by injecting encapsulating material into the first cavity.
 5. The method of claim 4 further comprising conducting encapsulating material through a channel from the first cavity to fill the second cavity.
 6. The method of claim 5 wherein: the encapsulating material is injected into a first end of the first cavity and conducted from the first cavity from a second end opposite from the first end.
 7. The method of claim 1 or 4 further comprising: providing a step-over cavity in the second mold section outside of the second cavity for accommodating features protruding from the second surface of the substrate.
 8. The method of claim 1 or 4 wherein the second region is smaller than the first region.
 9. The method of claim 1 or 5 wherein the channel comprises an aperture through the substrate.
 10. The method of claim 1 or 4 further comprising: providing a step-over cavity in at least one of the mold sections outside of the respective first or second cavity, the step-over cavity accommodating features protruding from the first or second surface of the substrate.
 11. A method for encapsulating two sides of a substrate comprising: providing a substrate having a first surface and a second surface; mounting a magnetic core to the substrate with a first portion of the core located in a first region of the first surface, the first portion extending above the first surface, and with a second portion of the core located in a second region of the second surface, the second portion extending above the second surface; providing an opening through the substrate; providing a mold including a first mold section having a first cavity for encapsulating the first region of the first surface of the substrate and a second mold section having a second cavity for encapsulating the second region of the second surface of the substrate; and providing a fill conduit for introducing encapsulating material into the first cavity.
 12. The method of claim 11 wherein the substrate comprises a printed circuit board.
 13. The method of claim 11 further comprising: closing the mold on the substrate; forcing encapsulating material under pressure through the fill conduit into the first cavity; and allowing encapsulant to pass from one of the first or second cavity to the other of the first or second cavity.
 14. The method of claim 11 further comprising: filling the second cavity with encapsulating material conducted through the opening in the substrate from the first cavity.
 15. A method for encapsulating two sides of a substrate comprising: providing a mold including a first mold section having a first cavity for encapsulating a first region of a first surface of the substrate and a second mold section having a second cavity for encapsulating a second region of a second surface of the substrate; using the substrate to seal the first cavity and the second cavity; providing an aperture in the substrate connecting the first and second cavities; providing a fill conduit for introducing encapsulating material into the first cavity at a first end of the mold; and flowing the encapsulating material from the first cavity through the aperture of the substrate into the second cavity. 